Ermakov–Lewis invariant

Many quantum mechanical Hamiltonians are time dependent. Methods to solve problems where there is an explicit time dependence is an open subject nowadays. It is important to look for constants of motion or invariants for problems of this kind. For the (time dependent) harmonic oscillator it is possible to write several invariants, among them, the Ermakov–Lewis invariant which is developed below.

The time dependent harmonic oscillator Hamiltonian reads

: $\backslash hat\{H\}\; =\backslash frac\{1\}\{2\}\backslash left[\backslash hat\{p\}^2+\backslash Omega^2(t)\backslash hat\{q\}^2\backslash right].$

It is well known that an invariant for this type of interaction

has the form

: $$

\hat{I}=\frac{1}{2}\left[ \left( \frac{\hat{q}}{\rho}\right)

^{2}+(\rho\hat{p}-\dot{\rho}\hat{q})^{2}\right],

where $\backslash rho$ obeys the Ermakov equation

:$$

\ddot{\rho}+\Omega^{2}\rho=\rho^{-3}.

The above invariant is the so-called Ermakov–Lewis invariant.{{cite journal | last=Lewis | first=H. R. | title=Classical and Quantum Systems with Time-Dependent Harmonic-Oscillator-Type Hamiltonians | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=18 | issue=13 | date=1967-03-27 | issn=0031-9007 | doi=10.1103/physrevlett.18.510 | pages=510–512}} It is easy to show that $\backslash hat\{I\}$ may be related to

the time independent harmonic oscillator Hamiltonian via a unitary transformation of the

form {{cite journal | last1 = Moya-Cessa | first1 = H. | author-link = Héctor Manuel Moya Cessa | last2 = Guasti | first2 = M. Fernández | title = Coherent states for the time dependent harmonic oscillator: the step function| journal = Physics Letters A| year = 2003 | volume = 311| issue = 1 | pages = 1–5| doi = 10.1016/S0375-9601(03)00461-4 | arxiv = quant-ph/0301111| bibcode = 2003PhLA..311....1M| s2cid = 54634409 }}

:$\backslash hat\{T\}=e^\{i\backslash frac\{\backslash ln\backslash rho\; \}\{2\}(\backslash hat\{q\}\backslash hat\{p\}+\backslash hat\{p\}\backslash hat\{q\}$

)}e^{-i\frac{\dot{\rho}}{2\rho}\hat{q}^{2}}=

e^{i\frac{\ln\rho}{2}\frac{d\hat{q}^2}{dt}}

e^{-i\frac{\hat{q}^{2}}{2}\frac{d\ln\rho}{dt}},

as

:$\backslash frac\{1\}\{2\}\backslash left[\backslash hat\{p\}^2+\backslash hat\{q\}^2\backslash right]=\backslash hat\{T\}\backslash hat\{I\}\backslash hat\{T\}^\{\backslash dagger\}.$

This allows an easy form to express the solution of the Schrödinger equation for the time dependent Hamiltonian.

The first exponential in the transformation is the so-called squeeze operator.

This approach may allow to simplify problems such as the Quadrupole ion trap, where an ion is trapped in a harmonic potential with time dependent frequency. The transformation presented here is then useful to take into account such effects.

The geometric meaning of this invariant can be realized within the quantum phase space. {{cite journal | last=Yeh | first=L. | title=Ermakov-Lewis invariant from the Wigner function of a squeezed coherent state| journal=Phys. Rev. A | year = 1993 | volume = 47| issue = 5 | pages = 3587-3592| doi = 10.1103/PhysRevA.47.3587 }}